Mixing device

ABSTRACT

A mixing device for intermixing liquids to a homogenous state. The device comprises a cylindrical housing and a generally cylindrical impeller rotatably positioned within the housing and includes a mixing section between the housing and impeller and means for recycling a portion of the mixture to the mixing section. They recycling of a portion of the flow provides improved homogenity of the final mixture.

United States Patent Harper [451 July 11, 1972 [54] MIXING DEVICE [72]Inventor: Robert C. Harper, Loveland, Ohio [73] Assignee: CincinnatiMilacron Inc., Cincinnati, Ohio [22] Filed: Dec. 15, 1970 [21 Appl. No.:98,277

[52] US. Cl. ..259/8, 259/D1G. 3O

[51] ..B0ll'7/24 [58] Field of Search ..259/7, 8, 23, 24, 43, 44, DIG.30, 259/9, 10

[56] References Cited UNITED STATES PATENTS 3,427,003 2/1969 Schneideret al. ..259/9 l/l965 Frenkel ..259/9X l/l969 Gurley ..259/7 PrimaryExaminer-Walter A. Scheel Assistant Examiner-Philip R. CoeAttorney-Howard T. Keiser and Alfred J. Mangels [57] ABSTRACT A mixingdevice for interrnixing liquids to a homogenous state. The devicecomprises a cylindrical housing and a generally cylindrical impellerrotatably positioned within the housing and includes a mixing sectionbetween the housing and impeller and means for recycling a portion ofthe mixture to the mixing section. They recycling of a portion of theflow provides improved homogenity of the final mixture.

12 Claims, 5 Drawing Figures INVENTOR. ROB T HARPER SHEET 1 BF 3FPYENTEUJUL 1 1 1912 SHEET 3 OF 3 MIXING DEVICE BACKGROUND OF THEINVENTION This invention relates to mixing devices and more particularlyto mixing devices of the substantially cylindrical type incorporating animpeller rotatably positioned in a cylindrical housing.

It is well known to mix several materials in the form of viscous liquidsin a shear-type mixer. Such a mixer frequently comprises a cylindricalchamber or housing within which a rapidly rotating impeller ispositioned. The impeller can be of a cylindrical type and frequentlyincludes spiral grooves on the outer periphery thereof to pump thematerials in the intended flow direction. The mixing takes place withinthe spiral grooves in the impeller and in the clearance space betweenthe housing and the impeller.

Oftentimes, and particularly when relatively viscous materials are to beintermixed, the output of such mixing devices is of a non-uniformcharacter in that the resulting mixture is non-homogenous. Morespecifically, it has been found that when mixing devices of the typehereinabove described are utilized for intermixing the components of atwo-part mixture for preparing foam plastics, the output of the mixerfrequently is a mixture which results in undesirable properties of theparts which are molded therefrom. For example, when polyisocyanates andpolyols are intermixed together with a foaming agent to provide a foamedpolyurethane article on curing of the mixed material in a mold, theresulting molded parts frequently exhibit severe streaking, soft spots,blisters, and non-uniform density. In order to provide the degree ofmixing necessary to overcome those conditions, it has been found thatthe through-put rate of material through the mixer has to be severelyreduced.

Increasing the speed of the impeller in such a mixer to permit anincrease in the flow rate was found to be impractical since it requiredexcessive power inputs to the system to achieve the degree of mixingdesired and caused considerable input of heat to the material. Thelatter condition is undesirable in that excessive heat will cause thefoaming agent in the mixture to vaporize prematurely, thereby causingfoaming to take place in the mixing head, where it is not desired,rather than in the forming mold, where it is desired.

It is therefore an object of the present invention to obviate theabove-described difficulties by means of an improved mixing devicepermitting a greater material through-put rate and which also provides amixture which when molded results in uniform, homogeneous articles.

SUMMARY OF THE INVENTION Briefly stated, in accordance with one aspectof the present invention, a mixing device is provided for mixing aplurality of liquid ingredients into a substantially homogeneousmixture. The device includes an inlet and a mixing section through whichthe material to be mixed passes and is mixed by shear forces and byturbulence. The mixing section is defined by the space between a pair ofrelatively rotatable, closely spaced cylindrical elements between whichthe materials to be mixed pass. A portion of the mixture which resultsafter passage of the components through the mixing section is returnedto the mixing section as recycle to improve the homogenity of theresulting mixture.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view ofthe mixing device of the present invention and illustrates a structurewhich provides the recycling contemplated by the present invention.

FIG. 2 is an exploded view of the mixing device of FIG. 1, partiallybroken away, to show the constructional details of the relativelyrotatable elements between which the mixing takes place.

FIG. 3 is a cross-sectional view of another embodiment of the presentinvention which incorporates a single mixing zone.

FIG. 4 is a cross-sectional view showing an additional embodiment of thepresent invention within which the recycling contemplated by the presentinvention can take place.

FIG. 5 is a cross-sectional view of a further embodiment of the presentinvention involving a modification of the structure shown in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand particularly to FIGS. 1 and 2 thereof, there is shown the mixingdevice of the present invention which comprises a cylindrical housing 11having an axial inlet opening 12 closed by a cover 13. Housing 11includes an outlet aperture 14 which is joined to the housing bodyitself by means of a transition section 15 of generally frusto-conicalconfiguration. Rotatably positioned within housing 11 is an impeller 16which comprises a central disc portion 17 to which are attached anupstanding annular wall 18 and a depending annular skirt portion 19.Impeller 16 is rotated by means of a drive shaft 20, which is connectedby known means to a source of rotational energy such as, for example, anelectric or hydraulic motor (not shown). Shaft 20 extends axiallythrough cover 13 and is keyed to disc portion 17 of impeller 16 by meansof key 21. Upstanding wall portion 18 of impeller 16 and drive shaft 20together define a first annular mixing section 22 into which thematerial to be mixed is introduced by means of metering tubes 23, 24,which extend through cover 13 and into first annular mixing section 22.The outer diameter of impeller 16 and the inner diameter of housing 11are so sized to provide a radial clearance space therebetween whichranges from about 0.010 inches to about 0.030 inches. The clearancespace and the spiral grooves in the housing and impeller define thesecond mixing section.

In the embodiment shown in FIGS. 1 and 2, a pair of metering tubes 23,24 is shown by means of which the materials to be intermixed can beintroduced into the mixer. However, it is not necessary that two suchtubes be provided. In fact, the number can range from one to up to sixor more, depending upon the number of streams of material to be mixed inthe device. If desired, a preliminary mixing operation can be performedexternal to the device shown in FIGS. 1 and 2 and the latter utilizedfor the final mixing step in a multi-step mixing operation. In thatevent, only a single metering tube would be required. Additionally,although the metering tubes 23, 24 are shown positioned for axialintroduction of the material to be mixed, it is not absolutely necessarythat that particular orientation be employed and the material can beintroduced in any direction as, for example, radially through thehousing 11 opposite the first annular mixing section 22. (See FIG. 3).

Impeller 16 includes a plurality of helical grooves 25 on its outermostsurface. Grooves 25 are preferably of a generally semi-circular crosssection and include generally triangularly shaped, rounded edges 26 onthe intervening ridges 27. The general arrangement of grooves 25 on theexterior surface of impeller 16 is shown most clearly in FIG. 2.

Referring once again to FIG. 1, upstanding annular wall 18 is of such aradial thickness that grooves 25 extend at least partially therethroughin order to permit communication between first annular mixing chamber 22and the radial clearance space between impeller 16 and housing 11.However, the radial thickness of depending annular skirt 19 issufficiently large to preclude grooves 25 from extending therethrough,thereby defining a plurality of helical flow channels of generallysemi-circular cross section.

Although shown as being of substantially semicircular cross section,grooves 25 in impeller 16 can be of any desired cross section such as,for example, rectangular, triangular, trapezoidal, or the like.Additionally, the helix angle which grooves 25 make with the axis ofimpeller 16 is preferably about to although helix angles within therange of from about 30 to about are also suitable. Furthermore, althoughin the embodiments shown in FIGS. 1 and 2 impeller 16 includes a totalof 8 equally spaced helical grooves, the number of such grooves is notcritical and any desired number can be utilized. However, although thereis some latitude with respect to the size and spacing of the severalgrooves, in order to provide the desired pumping action on the materialto be mixed it is normal practice that the grooves be so oriented thatthey define a left hand thread when impeller 16 is caused to rotate in aclockwise direction as viewed from above in FIG. 2. Similarly, ifimpeller 16 is rotated counter-clockwise as viewed from above in FIG. 2,the grooves normally are so oriented that they define a right handthread.

As shown in FIGS. 1 and 2, housing 11 includes a continuous helicalgroove 28 on its innermost surface to define a continuous flow channel.This flow channel and the intervening portions of inner side wall ofhousing 1 l are of generally trapezoidal cross section since the groove28 in the configuration shown in essentially an internal acme thread forconvenience of manufacture. Groove 28 can be provided along the entireinner surface of housing 11, but preferably it extends fromfrusto-conical transition section 15 upwardly to a point below firstannular mixing chamber 22. The helix angle of groove 28 relative to atransverse plane through the axis of housing 11 can range from 0.5 to60, and preferably ranges from about 1 to about Although shown as beingof substantially trapezoidal cross section, groove 28 can be of anyother desired cross section such as, for example, semicircular,triangular, rectangular, or the like. Preferably, the radial depth ofgroove 28 ranges from 0.02 to 0.2 times the inside diameter of thehousing. However, in order to provide the benefits of the presentinvention, the hand of the grooves in the housing side wall must be suchthat the rotational direction of motion imparted by the impeller to thematerial being mixed will induce an upward flow direction to thematerial in the grooves in the housing side wall. Thus, if when viewedfrom above as in FIG. 2 impeller 16 rotates in a clockwise direction,grooves 28 in the housing side wall should define a left handprogression for optimum operation of the device.

If desired, impeller 16 can be formed without upstanding skirt 18 tothereby eliminate first mixing section 22 shown in FIGS. 1 and 2. Theresulting embodiment would have but a single mixing section defined bythe space between impeller 16 and housing 11 as shown in FIG. 3. Thatstructure could be employed, for example, when the process in which themixing device of the present invention is utilized includes an external,preliminary mixing step.

In operation, the components to be intermixed are introduced through theseveral feed tubes 23, 24 (FIGS. 1 & 2) into first annular mixingsection 22. The rotation of impeller 16 will cause some degree of mixingto take place in mixing section 22 and as the material accumulatestherein, the centrifugal force imparted to the material by the rotationof impeller 16 forces the material radially outwardly and through thoseportions of grooves 25 in impeller 16 which extend through upstandingannular wall 18. Some mixing of the materials takes place in the courseof their passing through the radial apertures defined by grooves 25 inupstanding annular wall 18 and subsequently recombining in theintervening space between impeller 16 and housing 11. Other means ofpre-mixing the individual components prior to entering the primarymixing zone can also be used, or the individual components may be feddirectly to the primary mixing zone between impeller 16 and housing 11without pre-mixing.

Since the grooves in impeller 16 are normally so oriented with respectto the direction of rotation of impeller 16 that a downward component ofmotion is imparted to the material to cause the same to flow generallyaxially through the device toward the outlet aperture 14, the result isa pumping action which causes the material to flow downwardly. Thisdownward flow can also be accomplished by the supply pressure of theindividual components forced into the mixing device in the event thatthe impeller does not impart a pumping action. As the material flowsdownwardly through grooves 25, shear forces are imparted to the materialby virtue of the relative rotation between the housing and the impeller.Those shear forces serve to further inter-mix the material both bylaminar shear in the close clearance zone between housing 11 andimpeller 16 and by turbulence set up within grooves 25. The mixedmaterial then flows through transition section 15 and outlet aperture 14and can then be deposited in a suitable receptacle for furtherprocessing. For example, the mixer hereinabove described is particularlysuitable for use as a mixing device in connection with the preparationof polyurethane foams. In that case, one of the components to beintermixed as a polyisocyanate compound and the other component is apolyol. One of the components includes a foaming agent which can beactivated by heat or chemical reaction in a known fashion and whichcauses the liberation of gas, thus forming a multiplicity of cells inthe mixture, which when reacted becomes a rigid structure. Thosematerials are of a relatively viscous nature being on the order of l,00020,000 centipoises. Although the use of the device in conjunction withthe mixing of components to form a polyurethane foam has been disclosed,it would be apparent to those skilled in the art that other liquidmaterials including, for example, epoxy or polyester resins can also beintermixed by using the present invention. Furthermore,quasi-homogeneous liquids such as emulsions and suspensions can also beintermixed by using the present invention.

Groove 28 provided in the inner wall of housing 11 materially aids inmixing the components to a substantially homogeneous state in that itpermits recycling or recirculation of material by upward orcounter-current flow through the mixing device. The flow of the materialin grooves 28 of housing 11 is upward because of the orientation of thegroove with respect to the direction of rotation of impeller 16. Thus,that particular orientation provides a reverse pumping action, orfeedback, as compared with the downward pumping action of the grooves inthe impeller side wall. In providing the feedback through the mixingdevice, the described embodiment provides a more homogeneous mixturethan would be the case if the grooves in the housing side wall were notprovided. As a consequence, the throughput rate of materials to providea given degree of homogeneity can be considerably increased by virtue ofthe improved mixing which takes place when the mixer of the presentinvention is employed. In essence, the grooves in the housing provide anupward path for a portion of the mixed or partially mixed material to bereturned to the beginning of the primary mixing zone between impeller 16and housing 11. In this manner, material in the mixing device is mixedwith material that has entered the device at a later time. In effect,the residence time of the material is increased and short term errors inthe ratios of ingredients are averaged to a homogeneous mixture byvirtue of the configuration of the device. As a result, it is notnecessary to reduce the throughput rate to achieve a homogeneousmixture, which would be the case if the grooves in the housing were notprovided.

Another embodiment of the present invention is shown in FIG. 4. In thatembodiment the mixing device incorporates substantially the sameelements as shown in the embodiment of FIG. 1. However, in theembodiment of FIG. 4, housing 11 does not include groove 28 of FIGS. 1and 2 and thus the inside wall of housing 11 is a smooth, cylindricalsurface. The embodiment of FIG. 4 includes a baffle tube 29 positionedcoaxially with outlet aperture 14 and which extends upwardly into thearea enclosed by depending annular shirt 19 of impeller 16. Skirt 19includes a plurality of radial apertures 30, which can be provided in asingle transverse plane. Apertures 30 provide communication between thespace enclosed by skirt l9 and the intervening space between the outersurface of impeller 16 and the inner surface of housing 1 1.

In operation, the entering material is intermixed in first annularmixing chamber 22 in a manner similar to that described in conjunctionwith the embodiment of FIGS. 1 and 2, and also travels axially betweenimpeller 16 and housing 11 in a similar fashion but without the reverseflow provided by groove 28 in housing 11 shown in FIGS. 1 and 2. Afterpassing around the lower portion of depending skirt 19 of impeller 16,the material flows upwardly between depending skirt l9 and baffle tube29. In flowing upwardly the material passes radial apertures 30 and byvirtue of the centrifugal force imparted to the rotating material,causing it to be urged against the inner surface of depending skirt 19,a portion of the material passes radially outwardly through apertures 30and joins the material which is progressing downwardly between impeller16 and housing 11 in the mixing zone therebetween. Thus, radialapertures 30 provide a feedback means whereby some recirculation isprovided to further improve the mixing capabilities of the discloseddevice.

Preferably, in order to insure that some material passes through radialfeedback apertures 30, baffle tube 29 is positioned so that its topmostsurface lies in a plane which passes through radial apertures 30, or ina transverse plane which is axially upwardly spaced from apertures 30.This orientation of the several elements will insure that the materialpasses the apertures, and thus a portion thereof will flow throughapertures 30 to be mixed with material which is passing between impellerl6 and housing 11.

' A further embodiment of the present invention is shown in FIG. 5. Thatembodiment is similar in structure to that of FIG. 4 except that theinner surface of depending skirt 19 slopes upwardly and outwardly tofacilitate flow of the mixture to radial apertures 30 by centrifugalaction. The centrifugal action forces the material against the innersurface of skirt l9 and the slope thereof causes the material to flowupwardly toward apertures 30. Some of the material passes throughapertures 30 while the remainder passes through baffle tube 29.

it can thus be seen that the present invention provides an improvedmixing device for liquid materials by permitting recirculation of aportion of the already mixed material to join some that has not beencompletely mixed, thereby improving the homogeneity of the finalproduct.

Further mixing can be provided by including a screen in first annularmixing section 22 of the embodiments of FIGS. 1, 2, 4 and 5. Such ascreen is shown and described in the copending application of DonaldDunn, entitled Process for Mixing Coreactive Liquids Which FormPolyurethanes, Ser. No. 878,081, filed Nov. 19, 1969, and assigned tothe assignee of the present invention.

While particular embodiments have been illustrated and described, itwill be apparent to those skilled in the art that various changes andmodifications can be made and it is intended to cover in the appendedclaims all such changes and modifications that are within the scope ofthis invention.

What is claimed is:

l. A mixing device for providing a homogeneous mixture of a plurality ofliquid materials, said device comprising:

a. a mixing section having an inlet and an outlet and defined by thespace between a pair of relatively rotatable, closely spaced cylindricalelements between which said materials pass and are mixed by shear andturbulent forces, said mixing section including a cylindrical housingand a closely fitting cylindrical impeller having at least one helicalgroove on its exterior surface;

b. recycle means comprising at least one helical groove formed on theinner surface of said housing for returning to said mixing section aportion of the output therefrom to improve the homogenity of saidmixture, said helical groove being of the same hand as the helicalgroove on said impeller;

c. means for introducing the liquids to be mixed into said mixingsection; and

d. means for rotating said impeller in a direction of rotation to causesaid helical grooves in said impeller to impart to said material acomponent of motion in a direction toward said mixing section outlet.

2. The device of claim 1 wherein said helical groove on the innersurface of said housing is formed in a major portion thereof.

3. The device of claim 1 wherein said helical groove on the innersurface of said housing is of a smaller radial depth than that of saidimpeller groove.

4. The device of claim 1 wherein said helical groove on the innersurface of said housing is equal in radial depth so that of saidimpeller groove.

5. The device of claim 1 wherein said helical groove on the innersurface of said housing is greater in radial depth than that of saidimpeller groove.

6. The device of claim 1 wherein the helix angle of the helical grooveon the inner surface of said housing is greater than the helix angle ofthe helical groove on said impeller.

7. The device of claim 1 wherein said helical groove on the innersurface of said housing is of a substantially trapezoidal cross-sectionand the intervening ridges as of substantially trapezoidalcross-section.

8. The device of claim 1 wherein said helical groove on said impeller isof substantially semi-circular cross-section and the intervening ridgesbetween adjacent grooves are substantially V-shaped.

9. The device of claim 1 wherein said helical groove on said impeller isof substantially trapezoidal cross-section and the intervening ridgesare also of substantially trapezoidal crosssection.

10. The mixing device of claim 1 wherein said device includes apremixing section positioned within the confines of said cylindricalhousing into which said liquids are introduced and intermixed prior toentering a second mixing section defined by the space between saidcylindrical elements.

11. The device of claim 10 wherein said first mixing section comprisesan upstanding annular wall on said impeller defining an annular basketmixing section into which said materials are deposited and from whichsaid materials are expelled by means of centrifugal force, saidupstanding annular wall including a plurality of radial aperturestherein to provide communication between the annular basket and thespace between the housing and the impeller.

12. The device of claim 11 wherein said helical groove on the exteriorsurface of said cylindrical impeller extends into the region of saidannular basket and through said upstanding annular wall to define saidapertures.

1. A mixing device for providing a homogeneous mixture of a plurality ofliquid materials, said device comprising: a. a mixing section having aninlet and an outlet and defined by the space between a pair ofrelatively rotatable, closely spaced cylindrical elements between whichsaid materials pass and are mixed by shear and turbulent forces, saidmixing section including a cylindrical housing and a closely fittingcylindrical impeller having at least one helical groove on its exteriorsurface; b. recycle means comprising at least one helical groove formedon the inner surface of said housing for returning to said mixingsection a portion of the output therefrom to improve the homogenity ofsaid mixture, said helical groove being of the same hand as the helicalgroove on said impeller; c. means for introducing the liquids to bemixed into said mixing section; and d. means for rotating said impellerin a direction of rotation to cause said helical grooves in saidimpeller to impart to said material a component of motion in a directiontoward said mixing section outlet.
 2. The device of claim 1 wherein saidhelical groove on the inner surface of said housing is formed in a majorportion thereof.
 3. The device of claim 1 wherein said helical groove onthe inner surface of said housing is of a smaller radial depth than thatof said impeller groove.
 4. The device of claim 1 wherein said helicalgroove on the inner surface of said housing is equal in radial depth sothat of said impeller groove.
 5. The device of claim 1 wherein saidhelical groove on the inner surface of said housing is greater in radialdepth than that of said impeller groove.
 6. The device of claim 1wherein the helix angle of the helical groove on the inner surface ofsaid housing is greater than the helix angle of the helical groove onsaid impeller.
 7. The device of claim 1 wherein said helical groove onthe inner surface of said housing is of a substantially trapezoidalcross-section and the intervening ridges as of substantially trapezoidalcross-section.
 8. The device of claim 1 wherein said helical groove onsaid impeller is of substantially semi-circular cross-section and theintervening ridges between adjacent grooves are substantially V-shaped.9. The device of claim 1 wherein said helical groove on said impeller isof substantially trapezoidal cross-section and the intervening ridgesare also of substantially trapezoidal cross-section.
 10. The mixingdevice of claim 1 wherein said device includes a premixing sectionpositioned within the confines of said cylindrical housing into whichsaid liquids are introduced and intermixed prior to entering a secondmixing section defined by the space between said cylindrical elements.11. The device of claim 10 wherein said first mixing section comprisesan upstanding annular wall on said impeller defining an annular basketmixing section into which said materials are deposited and from whichsaid materials are expelled by means of centrifugal force, saidupstanding annular wall including a plurality of radial aperturestherein to provide communication between the annular basket and thespace between the housing and the impeller.
 12. The device of claim 11wherein said helical groove on the exterior surface of said cylindricalimpeller extends into the region of said annular basket and through saidupstanding annular wall to define said apertures.